The use of automated drilling methods is becoming increasingly common in drilling subterranean wellbores. Such methods may be employed, for example, to control the direction of drilling based on various downhole feedback measurements, such as inclination and azimuth measurements made while drilling or logging while drilling measurements.
One difficulty with automated drilling methods (and directional drilling methods in general) is that all directional drilling tools exhibit tendencies to drill (or turn) in a direction offset from the set point direction. For example, when set to drill a horizontal well straight ahead, certain drilling tools may have a tendency to drop inclination (turn downward) and/or to turn to the left or right. Exacerbating this difficulty, these tendencies can be influenced by numerous factors and may change unexpectedly during a drilling operation. Factors influencing the directional tendency may include, for example, properties of the subterranean formation, the configuration of the bottom hole assembly (BHA), bit wear, bit/stabilizer walk, an unplanned touch point (e.g. due to compression and buckling of the BHA), stabilizer-formation interaction, the steering mechanism utilized by the steering tool, and various drilling parameters.
In current drilling operations, a drilling operator generally corrects the directional tendencies by evaluating wellbore survey data transmitted to the surface. A surface computation of the dogleg severity (DLS) and gravity toolface of the well is generally performed at 30 to 100 foot intervals (e.g., at the static survey stations). While such techniques are serviceable, there is a need for further improvement, particularly for automatically accommodating (or correcting) such tendencies downhole while drilling; thus controlling the dogleg severity and toolface in a closed-loop manner.